International Concert Pitch
In 1939, an international conference recommended that the A above middle C be tuned to 440 Hz, now a standard known as International Concert Pitch. This standard was taken up by the International Organization for Standardization in 1955 (and was reaffirmed by them in 1975) as ISO 16.
The difference between this and the diapason normal at 435 Hz is due to confusion over which temperature the French standard should be measured at. The initial standard was A = 439 Hz, but this was superseded by A = 440 Hz after complaints that 439 Hz was difficult to reproduce in a laboratory owing to 439 being a prime number.
- William Vennard (1967). “Pitch”, Singing: The Mechanism and the Technic, 5th ed. Carl Fischer, LLC, p. 3. ISBN 0825800552.
- George Ashdown Audsley (1965). [http://books.google.com/books?id=E-PckX9PPfwC&pg=PA636 The Art of Organ-building: A Comprehensive Historical, Theoretical, and Practical Treatise on the Tonal Appointment and Mechanical Construction of Concert-room, Church, and Chamber Organs, Volume 2], Reprint of 1905 ed. Courier Dover Publications, p. 636. ISBN 0486213153.
- Lynn Cavanagh. A brief history of the establishment of international standard pitch a=440 hertz (PDF). WAM: Webzin o audiju i muzici. Retrieved on 2012-06-27.